Telemetering system



'7 Sheets-Sheet 1 O. H. SCHMITT ET AL TELEMETERING SYSTEM June-12, 1951 Filed May 25, 1948 INVENTORS BY 55u; @fr ffl-7 M ATTOREYS.

J'une l2, 1951` o. H. SCHMITT ET AL TELEMETERING SYSTEM Filed may 25, 194s l l G/' l 7 Sheets-Sheet 2 OTTO WIN IRA ELD WESLEY N'YENTORS A. F KASINDORF TTORN lIYS June 12', 1951 Q H, sCHMn-T ET AL 2,556,556

TELEMETERING SYSTEM Filed May 25, 1948 7 Sheets-Sheet 5 FIG. 3.

A A A A A A A A A A A A 0 30 60" 9 130 150 /80 1?/0" 240Z70 300' 330 360 RESULTANT FIELD 4 c`z` C I. 9 CLD) 900 Lof/E5 o (D O O A A L A A A A L A A L 0 30' 60" 90 l/EO7 [50 [60 FIO" F40 @70' 300" 330 360 ROTOR POSITION :Inventors OTTO H. SCHMITT WINFIELD E. FROM" WESLEY A. FAILS 13B IRA L. KASINDORF GIIornegS June l2, 1951 o. H. SCHMITT ET AL TELEMETERING- SYSTEM '7 Sheets-Sheet 4 Filed May 25, 1948 ATTORNE June 12, 1951 Q SCHMlT-r ET AL I 2,556,556

TELEMETERING SYSTEM Filed May 25, 1948 7 sheets-sheet 5 FIG. 6.

9 I "D INVEN'roRsV oTToIEH. scmwr'r` WESLEY A. FAILS BY IRA L. KASINDORF June 12, 1951 o. HfscHMlTT ET AL v TELEMETERING SYSTEM 7 Sheets-Sheet 6 Filed May 25, 1948 wv um,

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INVENTORS WESLEY AE BY 64 lRA L. KSNDORF (ng/2.443- -#ZnL-:Ly

ATTORNTPYQd I June 12, Q H- SCHMITT ET AL TELEMETERING SYSTEM Filed May 25, 1948 '7 `Sheets-Sheet 7 ln. l

INVENTORS OTTO H. SCHMITT WINFIELD E. FROMM WESLEY A. FAILS lRA L. KASNDORF @hr3/a7 vom( ATTORNEYS Patented `lune 12,A 1951 field E. `Fromm;rjiillistor, Wesley Fails, Hempsteadyand Ira L. Kalsindorf,l Bronx, N; Y.,

assignors to Airborne I Incl, Mineolafl.'

nstrui'nents' Laboratory,

Applicationltilay 25v, 1948, Serial No. 29,91?

VThis invention relates to telemetering systems and 'more particularly to the transmission .and

reception of angular and magnitude .information.

In accordance with the present invention polar coordinate data are transmitted by modication of thewave characteristics of a single Acarrier 'afrequency; accurate'relaying of the data b eing acsclaiels. (C1. 1777-351) complished without the use .oi precision vtransmitting andreceiving equipment.

Many systems have been proposed for the telemetering of angular information, but all have failed to provide the desired characteristics of accuracy and 4economy of equipment' 'For example, it has been proposed to transmit angular information by coding the frequency of a 'carrier wavejinaccordance With the angular information to be telemetered.A Such Va system of frequency modulaticnfrequires precision transmission and reception equipment'which is expensive to construct and requires continued and Vskillful maintenance, and relatively complicatedapparatus is requiredfor converting the electrical signal; into a 'formjsuitable for automatic controlling orrecording operations.

It @has also been proposed to modulate ,the carrier vfrequency with van audio signal the Vfrequency of which is coded in accordance with lthe angular information to be telemetered. Systems of this type also require the use of precisionfrequeney standardization equipment at the :transmittihg and receiving stations. The electrical information derived `from both of these systems requires extensiveemodication before it can be used for automatic controlling or recording purposes' In accordance with the preferred embodiment of this invention two voltages derived'f-for example, from two voltages of a Selsyn system, 'are used to control the side-band frequencies of a carrier Wave'. With this arrangement theaccuracy of transmission is largely dependent upontlfe ratio ofthe side-band frequencies so that absolute `frequency standards are not required for either vthe transmitting or the receiving stations. The three Selsyn voltages are reconstructed at the receiving station and thus may be, used readily ,for4 control operations. A third frequency may be generated corresponding, for example, to the magnitude data portion of the polar information and transmitted simultaneously on the same carrier.

Accordingly, it is an object of this invention tolprcvilemethods and apparatus for.te!@meteri .mamar impresion.

:It is a further objectY to provide such a system invvhichftheaccuracy of the'system' is in alarge measure independentn of the frequency andfampl-:iitilde'l stability of n the transmitting and receiving A.. Si; i

`$til1janother object is to provide for the trans mission of angular information by transmittinga carrier wave modified in accordancewithth'e instantaneous `magnitudes of two" of the Athree contrl voltages of aSelsyn system. i

` It' is another object top rovide novel component equipments suitable for uselin Vsuch ka -systemiand havingwide utility for other'applications;l

The invention accordingly consists in the features of construction, combinations of' elements,

arrangements' of'partsan'd methods of operations alsyvin4 "be exemplified in the structures'l'andseque'rlc'es and 'series of steps "to' "be hereinafter indicated 'and 'the scope 'of the'applicati' lf which 'Will be setffortl'f'nthe" OllOivV'ngblaliInS.

1n Vthis specification andthe accompanying drawings, lthere i's` shown"and"descr`ibe'd `a prferred embodiment of theinven'tionfbiit itfis'to b' understodthat this anni uiten-ded' to fte egihaustive nor limiting of the invention, but'n the'` ccntraryiisl given for purposesl of illustration in order :that others skilled in theartrnaylfully liriders'iaad the intenti@ arid the Drps tlieref andthe .manner 0f applying lt'iri lere@- tiqal use .S0 ihatthey .may mqdifyald adapt' it in various forms, each -as maybe best suitedto theqccnditionsof aparticular use. Y

ln theidrawingsr ,.Flig- ..1 .is .a bleek diagram ef l a System, ,991.1- stuted "in accordance with the presentnrehnearer the teler-@ienne Qfpqlariefermaiien;

rig. 2 ,shows .schematically-apparatus f ribducirig en audio modulation i1eeiueu9y-na r l anqawith appliedelsynveltases Fia .Shows the .voltages .Offre Selena es a functinfih .ireciiori Qfihefrs .Peet eld;'

,llashcws @Gewisse-ceding..modulation fliegueeeiesss .a function. @ft-hemd direfie:

vvFig. 5 is a circuit diagram of apparatushfor rpro- 1 ping 'a montantsfr'queh'cy r'accbrdiice Withjanlapplied direct voltage;A

Fig. 6`is a schematic diagram of a mixenV ampliiier'fo combining the modulation volta"A Y Fig. 'lisk a partialschematic:diagram of ,fthe criniinators and inverters utilized in the receiving equipment for' reeonv'ertingr` the modulation sigr'ialsY into alternating Selsin voltages; andA` "8` showsY 'the amplier vfor' 'tlese voltages and apparatus for reconstructing the third Selsyn voltage.

SYSTEM OPERATION The block diagram of Fig. 1 represents the entire apparatus for telemetering angular information from a Selsyn system, and for simultaneously tclemetering corresponding magnitude data so that a complete polar plot may be made automatically at the receiving point. In order to transmit a signal S-I, which is a function of the angle to be relayed, the three alternating voltages from a 60 cycle Selsyn system may be used. These voltages may be taken conveniently from a Selsyn control system, as for example, one which indicates direction, and which may be generated automatically by a gyro-compass system. It is, however, unnecessary to transmit all three of these voltages because, in the usual Selsyn system, the sum of the three voltages is at all times equal to zero. Therefore, knowing the values of any two of the three voltages, the third one can be determined. The signal S-I is applied to the eld windings of a servomotor B. In order to accurately position the rotor of servomotor B, in accordance with the signals S-I, without undesirably loading the voltage l source A, the voltage induced in the rotor winding of motor B is applied to an amplier C the output of which drives a two-phase motor D which accurately positions the rotor of motor B and simultaneously positions two Selsyn motors E and F.

These Selsyn units E and F are connected in the circuits of two variable frequency oscillators G and H, respectively, and control the frequency of these oscillators in accordance with the instantaneous angular position of the armature of motor D. A third oscillator J, also variable in frequency, is controlled by a signal S-2 which may be direct voltage, derived in any suitable manner, the magnitude of which is to be transmitted as part of the polar coordinate information.

The output signals from oscillators G, H, and J, al1 of which may operate conveniently within the audio frequency range, but which occupy separate frequency channels that do not overlap, are combined in a mixer-amplifier K the output of which is utilized to amplitude modulate a conventional type radio transmitter L.

The transmitted signal is picked up by a conventional type receiver N and, after suitable amplincation, is detected and the output signals applied simultaneously to three filter circuits: a high-pass filter O which suppresses the modulation frequencies produced -by oscillators J and H but does not appreciably attenuate the frequencies produced by oscillator G, a band-pass lter P which passes only the frequencies produced by oscillator H, and a low-pass filter Q which passes only the frequencies produced by oscillator J. These lters are connected, respectively, to discriminators R, T, and U, each of which delivers direct voltage having a magnitude that is a function of the modulation frequency applied to the particular discriminator, and which is substantially independent of the magnitude of the modulation frequency.

The output voltages of discriminators R and T are applied to inverter circuits W and X, respectively, each of which produces an alternating voltage of predetermined constant frequency and having a magnitude which is a function of the direct voltage delivered by the corresponding dis-- criminator.

These alternating voltages are applied to the input circuit of an amplifier Y which amplifles the two voltages and which produces automatically a third voltage corresponding to the third component of the original signal S-I and which was not transmitted. This signal is applied to a servo control motor Z which may control, for example, the angular position of a turntable of an automatic recording device.

'Ihe signal from the low-pass filter Q passes through a similar discriminator U and the direct voltage therefrom, the magnitude of which is a function of the frequency of oscillator J, is applied to a suitable control circuit, for example, the automatic recording device mentioned above, to control the radial position of a recording pen V.

Thus, data collected at one point; may be recorded accurately in polar coordinate form at a remote location and the transmission may be accomplished without the necessity for precision transmission and receiving equipment. Thus the transmitter L and the receiver N may be conventional types such as are used ordinarily for communication purposes, with no unusual stability requirements either as to power, sensitivity or frequency, because the data recorded at the remote location are independent of these factors.

TRANSMISSON EQUIPMENT Adjustment of synchro units E and 'F The three voltages from a suitable source A, for example, a Selsyn system, which denote the angular information to be remotely indicated, are applied, respectively, to stator windings 2, 4, and 6 of a synchro motor B. (Fig. 2.) These three windings are arranged physically 120 degrees apart to produce a resultant magnetic eld the direction of which is a function of the relative magnitudes of the three voltages. The effective values of these voltages and the direction of the resultant field produced thereby are shown in Figure 3. The curves E2, E4, and E6 represent the effective values of the alternating voltages, 60 cycles in this example, between the terminals of respective windings of the synchro motor B. These voltages, upon which the direction of the resultant eld depends, are in phase as regards time, except that crossing of the zero line in Fig. 3 indicates a reversal of phase, and thus differ only in relative magnitudes and directions of current flow. A voltage is induced in rotor winding 8 of synchro motor B dependent upon the voltages applied to the stator windings 2, 4, and 6, and upon the angular position of rotor 8 with respect to the angular direction of the resultant field.

The output of rotor winding 8 is connected, through a 4 to l step-up transformer I2, to a phase shift circuit comprising a condenser I4 and a resistor I6. This circuit shifts, by 90 degrees, the phase of the 60 cycle voltage from the Selsyn system so that after suitable amplification it may be used for operating the two-phase induction type reversible motor D.

The voltage appearing across condenser I4 of the phase shift circuit is applied through a condenser I8 to a grid 22 f a triode vacuum tube 24. This tube is cathode coupled through a cathode resistance 26 to a similar tube 28 to form a phaseinverter amplier circuit. Because of the high value of cathode resistance 26 required in this circuit, it is necessary to apply positive voltage to die "grid 522 or tube 2a and gris 32 of time f2s.. This toitagis provides ffrm a =siiitab1e '-sower lsur fin tjshown) through a positive voltage fsupplyjllead131i, ibynieansfia voltage divider circuitfcornprisinglresistors'3S andf'38; the-grids being connected ttofa point 'between these two resistances'by*gridjreturnresistors 42 and 44. The :anodes "lidian'dil of tubes '24 vand 28, respectively, 'are coupled jby conventional resistance-capacinetworls rto ush-pull'output circuitinjludin'g `'vacuum tubes *52 and 5d and an'output transformerf55- r Y l 1 Thefsecondary winding 58 or out-put transform- Gil/is coupledtoiield Iwinding 62 Yof the motor The otherwindi'rg 64 of this 'two-phase motor isfconnected to asourcejof alternating voltage 'that fispin .phase with :the 4-Selsyn voltages which Aare applied rto 5the stator windings'ofsynchro motor iB. The voltage applied to 'winding S2, 1thus,1le;ads or lagsby 90"`degrees, the voltage ap- "pli'eldfto winding Sflfbeoause of the phase shift produced by condenser i4. Thus, the rotation of "the armature of motor D depends upon the magnitude of'the'current induced in rotor winding lBof "synchro rn'otor B; the direction of rotation depending upon 'whether'the 'voltage induced in l'winding"B is in phase, or 180 degrees out of phase, with the Selsyn supply voltage. 'I'he armature of motor D is connected mechanically to control lthe'angul'arpositicml of rotor winding' of synchro motor B and rotorwindingst'and 68 of' synchro 'units E and vl, respectively. The windings oi' moftrBla/re vconnected so that the Voltage induc'edlin the rotor winding 8 causes motor -D vtoadjust the rotor winding `8 in'such direction "as to 'reduce the voltage induced therein. Thus, when'vlta'ges arefap'plied 'tothe stator windings k2, Il, 'and' 6 'of synchro motor B,fmotor D rotates 'the rotor of motor vB until Zero volta'geis induced "in Winding.

However, because'of the inertia of the mechanical system and because Zero'voltage is induced 'in winding Bat the condition of balance, there is a tendency to overrun the zero position until sufficient torque is developed in the opposite di- 4reotiori'to stopl and'reverse'inotor D. This would v "cause oscillation or hunting in the'system. In

accordance ywith ithe'pr'esent invention, such hunting isprevented by atuned feedback circuitconnectred between the ungrounded sideof secondary windig of transformer '56 and the grid 32 `of` vacuum tube 28. This winding is connected 'throughyjanadjustable resistor l2, a fixed rejsistancefl'll, anda 'condenser 16 to the grid 32. fvAlso connected between the grid 32 andY ground fis-epara11e1 f circuit, including an inductsnce is 4arida capacitance 82, which is resonant at the applied* frequency. An adjustable resistance',

` When motor D movesthe'rotor Winding v'vof 'synchrofrnotor B to the position of Zero voltage,

there 'is no inputto the amplifier from that source. `There'is,`ho\vever, a negativefeedback voltage applied to the input of the amplier through condenserk 16, because the Voltage which is developed. across the tuned circuit is mainftained'ffor several cycles before the stored energy is dissipated, producing a relatively largel signal vin'fthe-Hop'osite f directionl from lthat which was delivered bythe winding-8; and applying a backward torque to motor D thereby damping the --and 6 hunting Iaction. By `adjusting Iresistance 12 v-to vary 'the jfee'dfb'ack ratio, land resistance 84 Lto vary `.the A:Q of ithe resonant circuit, 'the' damping may be made 'just sufcient to @prevent oscillation 'and provide La stable system for positioning the rrotor oi-:synchro inotor 5B and synchro Vunits E and 'F Awithout loading :the Selsynor other circuit vwhich supplies "the signal voltages-L oscillators vG and H Thus, the rotors of synchro units `E and F i are adjusted to Yanangular position-in accordance with the datasu-ppli'ed by the l Selsy/nisignal. S-j-v- I These synchro units are rincorporated vin the plate'circuitsof oscillators G andH,lrespecti`v'ely. and -ihaccordance with the angular positions -of the rc'tcr windings )66 f and 68 vcontrol the Lfrey duencie's f` of foscillationffof the respective To'scilla'- tors.

"Oscillator '"G, which, `furthe pur-poses of this example, is -assumed to 'operate over the-freoluency range iroinl/ll to 2950 cycles persecond, isshcwn indetailinlig. 2. Oscillator H (details not shown) is identical with oscillator G, except thatitis` designedto operate over the frequency range between yi300 vand 1200 cycles per second, and -Visy connected -in a similar manner to -th terminals 86, 88 of synchro unit F.

Synchro-unit E has the'usual stator iwindings S2, 94, and 95 disposed physically 120degrees apart, around the rotor `v`winding t5. lStator windings '92 andihl are connected 'infseriesand this 'series combination is connected in parallel with rotor "winding i @5. The total inductance/apnearing between output leads QSand 102 is th-us a function oflthe position of rotor Winding 66 relative to'fthe' stator windings 92 -and #94.

`In this lparticular application 'it/is 'desirable "for'the value-'of this inductance, which lis Veffective' in the oscillatorG, to vary in such a manner that-theoscillator frequency is a sine function of theangularpositmn of rotor 66. To accomplish this, an inductance lil is placed in'pa'rallel fwithv` the output leads B8 and l2 and a second "inductance -lili isplaced in serieswith the'output 'circuit' thereof. The inductances 104 and are provided'with low-loss powdered Viron cores'which may be adjusted. in position relative 'to the turns ci' the coils, to change the inductance lof the respective coils. By proper adjustment 'of theinductances of these two lcoils the total `inductan`ce follows closely' a sine 'function of the angle of rotation or" rotor Winding S6.

v "condenserlil is connected in parallel with the u'inductive circuit just described tofform Ma `parallel circuit, the resonant frequency of which may be'varied by rotating the rotorwindingl of the 'synchrc'unit This parallel cicruit forms the resonant circuit' oi a bridge-type oscillator.

A varistor, `or negative coefiicientresistance elenieiit,"ll2 is connected in parallel with the resonant circuit. "lliisi'varistor is for the purpose of stabilizing the amplitude of the oscillator-circuit-as will loe-*described later.

The-parallel resonant circuit, together ywith the varistor H2, forms onearm of a bridge circuit. This arm balances against a resistance Yliiig--the'fthird and fourth arms of the bridge circuit being formed by resistancesl i6 and H8.

The supply voltage for thebridge is provided by anodes l22-and Yi2fl ofA triode vacuum tubes 126 t28, -respectively. The resistance YH8 is slightly lessin value than the resistance HS,

-sothat a slight unbalance condition exists and -a corresponding voltage exists between points [32 and |34 of thebridge circuit. This unbalance voltage is applied through a condenser |36 to grid |33 of vacuum tube |26. The anode |22 of this tube supplies operating potential to one side of the bridge circuit and is coupled also through a condenser |42 and a resistance |44 to grid |46 of vacuum tube |28. The anode |24 of this tube is connected to the opposite side of the bridge circuit so that the voltage provided for operation of the bridge circuit is the dierence in potential l existing between anodes |22 and |24. Thus, the unbalance voltage of the bridge is fed back through two stages of amplification and applied as the operating bridge voltage. The frequency of oscillation depends upon the parallel resonant i circuit described above and may be varied through approximately one octave by rotating the rotor winding 66 of synchro unit E through 360 degrecs.

The varistor H2 is chosen so that at the desired amplitude of oscillation its resistance, which is a function of the voltage existing across the varistor, is approximately equal to the impedance of the parallel resonant circuit. The total pedance of this arm of the bridge therefore, is a function of the resistance of the varistor which is in turn a function of the voltage existing across it.

As the amplitude of oscillation increases, the resistance of the varistor decreases, which, because of the values of resistance chosen for resistors ||4, and H6 and H3 reduces the unbalance or" the bridge and, therefore, reduces the unbalance voltage which is applied to the grid of vacuum tube |26 and which in turn decreases the voltage supplied to the bridge by the anodes of vacuum tubes |26 and |28. VIf the amplitude of oscillation decreases, the resistance of varistor ||2 increases, which creates a, greater unbalance in the bridge and, thus, applies a higher voltage to the grid |38 of vacuum tube |26, and so a higher voltage to the bridge circuit thus providing automatic compression.

The amplitude stability of this oscillator has been found very good, even though the Q of the synchro unit E is generally low. One particular oscillator, in which the Q of the synchro unit was between three and live, produced an average amplitude of ten volts and maintained this arnplitude within plus or minus three percent, as the frequency was Varied through one octave by rotating the rotor Winding @e of the` synchro unit E. At this ten Volts output level, ail harmonics were down at least decibels. Fig. i shows the curve of frequency Variation of oscillator G- and oscillator H in accordance with the angular position of rotor windings e6, and 68, respectively, which correspond to the angular information provided by the Selsyn voltages. The output circuits of oscillators G and H are connected to the input of the mixer-amplifier K.

Oscillator J The magnitude data which are to be transmitted are applied by a direct current signal 5 2, the magnitude of which is a function of the data, to the terminals |52 and |54 (Fig. 5) of the variable frequency oscillator J. Terminal |52 is connected through a switch arm |56, and resistances |53 and lSl to a control grid |62 of a pentode vacuum tube |34.

The current through the tube |64, as will be explained below, controls the frequency of the oscillator J and, therefore, must not be subject to Variations caused by factors other than the applied signal S-2. A dual diode |66, the two sections of which are connected in series and are in turn connected in parallel with the resistance |64, serves the dual function of providing negative bias for the tube |64 and compensating for changes in the filament voltage applied to tube |54. This compensation takes place in the following manner: when the filament or cathode of an ordinary vacuum diode is heated, there is electron emission from cathode to plate even though no positive voltage is applied to the plate. This action produces a negative voltage on the plate oi the tube, relative to the cathode. In the present application the voltage produced, by this effect, on anode |61 of tube |66 provides bias voltage for the tube |64. The filaments or heaters of tube |65 and tube |64 are connected to a common source so that a change in the voltage applied to the heater circuit of tube |64 simultaneously affects tube |66. Assume an increase in these heater `voltages which will tend to increase the plate current through tube |64. This increased heater voltage will increase also the electron emission of the cathodes of tube |66 thus placing an increased negative voltage on anode |51 of this tube and increasing thereby the negative voltage on the grid |62 of tube |64; thus providing automatic compensation for the increase in heater voltage. If the heater voltage decreases, compensation takes place by reducing the negative bias on tube |64.

As a further measure to maintain constant plate current through tube |64, the Voltage of screen |68 is regulated by a gaseous discharge type voltage regulator tube |12', which is connected to the positive voltage supply lead |14 through a dropping resistor |16. The screen |68 is, of course, more influential in holding the plate current constant in this type tube than is the Voltage of anode |18 and the plate current remains substantially constant, even with considerable variation in the plate voltage of the tube.

Voltage is supplied to the anode |18 through four varistors, |82, |64, 86, and |28, which are connected in series between the anode |18 and the plate supply lead |15. These varistors, the resistance of which is a function of the current through tube H54, are connected also in a phaseshift feed-back circuit of a two stage oscillator to control the frequency of oscillation.

Two pentode tubes |52 and |94 are connected as a cathode-coupled oscillator. Cathodes |96 and |98 of these tubes are connected to ground through resistances 252 and 224, respectively, and through variable resistance 206, common to both circuits, for balancing the plate current through the tubes. Voltage for screens 2|0 and 2l2 of these tubes is provided through lead 208 from the same regulated power supply that provided the screen voltage for tube |64. Resistors 2M and 2de are connected in series with screen grids 2|@ and 2 l2, respectively, and together with oy-pass condensers 2|8 and 222, are provided to suppress parasitic oscillation. Anode 224 of tube |22 is connected through series resistors 226 and 228 to the positive voltage supply lead |14. Anode 232 of tube |94 is connected through series resistors and 236 to supply lead |14. Resistors 226 and 234 are connected directly at the tube socket terminals, as are by-pass condensers 238 and 242, for suppressing parasitic oscillations in the plate circuits.

Control grid 244 of tube |92 is biased positively with respect to ground by a Voltage divider circuit comprising resistances 246i and 243 which are connected between. the` supply lead |14 and ground. A lead 252v is connected through a series resistor 253 to a point between the two resistances and to. grid. 244` through a suppressor resistance 254. Lead. 258 is connected to the same point between resistors 24d and 248 and supplies bias voltage. to control grid. 2.58 of tube |84 through suppressor resistance. 262.;

Tube |92 is coupled to tube |94 by a leadl 234 which is connected between cathode |98 and cathode |981.. A lampf 2.8.8'l is connected in. series with this lead for the purpose of stabilizing the amplitude of oscillation. Lamp 288 has a positive temperature coefficient and, as the amplitude of oscillation increases, the current through lead 2.34` rises. thus increasing the resistance of lamp 266 which decreases the cathode coupling and, thus, the amplitude of oscillation. Adjustment of the tapped resistance 288 provides direct current balance ancleliminates. the ow of direct current. through. theilampzl 23.8. The amplitude; of oscillation may be adjusted to a desired value by Varying the amount of feedback through adjustment of tapv 268. onv resistance. 236.

Energy is. fed back from the anode circuit of tube |944 throughtheadjustable tap 26.8 of resistance 2:53 to av phase-shift network, enclosed within the broken line 212, which applies the feedback signal. to the control grid. 244 of tube |92.

The phaseof the voltage delivered by the anode circuitof tube |34. is such that maximum` amplitude of' oscillation will occur if there is no phase shift.. iin. the circuit 2.12. This. phase-shift. circuit 212-, which includes a condenser 2.24, the four varistors |82, |84, |83, and |88, and a` condenser 216, is; frequency sensitive and there is only one frequency at'. which zero phase shift takes place. rIhe amount by which. thephase is shifted in this circuit varies rapidly on either side of this critical frequency; the oscillator, therefore, generates that frequency for which there is zero phase shift. This. frequency is given by the expression:

l f-arRo The frequency may be varied accordingly by simultaneously changing thecapacity of theY two condensers or by changingA the resistancer of the varistors. In this circuit the resistance is the value that is changed. Because the resistance of the varistors changes with. change in. current through them the frequency of oscillation may be controlled or varied,- accordingly, by changing the plate current through tube |84' and conse'- quently the directl current through the varistors. This current is controlled by the signal S-2 which isV app-lied to the control grid |62 of tube |84. The frequency at which the oscillator operates when there is no. input voltage, i. e, when switch arm |56 is connected to terminal 2.18',Y is adjustedby Varyinga rheostat'288'in the cathode circuity of tube |84; This oscillator, for the. pur.- posesof. this exampleis. assumed to operate over the frequencyy range from. 2002110. 40.0 cycles per second.

In order to.. indicate the frequency at. which the oscillator is1operatinga bridge circuit,V gen.- erally indicated at 282, is utilized to measure` the direct voltage applied to. the. oscillator control tube. |64. A voltage divider, consisting ofi resistances 284, 28.8, and 288 is( connected between switch arm |53` and". ground. A. tap on resistor 233 is. connected to control grid 292. of a triode vacuum tube 284--, which is coupled through its cathodey 298 to cathode 382: of a similar tube; 334 by means of; al common. cathode resistance. 228. Control. grid 3536l of tube 384 is connected to ground. Anodes. 33,2 and 3m of tubes 284'. and 304., respectively, areconnected. together through a balancing potentiometer 3|.2the adjustable tap of which is. connected tot the supply lead |124. The, bridge is initially balanced, With zerovr voltage applied. to grid` 292, by adjusting4 the' variabler. tap 3 I4.. Vol-tage applied to grid 292; unbalances the circuit by an amount indicated on; a. voltmeter 3|6 connected between the. two. anodes. This meter may be calibrated conveniently in frequency and? so indicate directly they frequency of oscillation.

The output circuit of oscillator Jv is coupled froml the anode circuitlof tube |92 through a con'- denser 3|8 and is connected to the input orY the mixer-amplifier K (Fig. 1.) where it iscombined with the signals from: oscillators G and H.

Mixer amplifier K The output' signals of the oscilla-tors Gi and H. andi the output; from oscillator J are combinedfin a, resistance. network. (Figi. 6)' compris'- ing. three` Variable. resistances: 322,. 324, andi 3262, and a xed resistance 328. The individual poten.- tiometers are. providedv tof permit approximately six.' decibels ofi independent adjustmentot the output. of each oscillator. In applications where the. characteristics of the'. radio transmitter and receiver are such that they discriminate against low. or higher ranges: of' frequency, an equalizer pad may be provided, for compensation. This equalizer pad, indicated" within the broken line 332, for use where: the radio transmitter and receiver discriminate against the lowaudio: frequencies, produces an. insertion loss of four decibels at 200 cycles; fourteen decibels at 400' cycles', and sixteen. decibels above 1000. cycles. The equalizer pad may be. switched into or out of the.- circuit by means of aswitch 3341. The' output circuit. of the equalizer pad 332 i's connected throughia potentiometer 3.3.6andi a coupling condenser 338. toagrid 342 of triod'e tube'3'44l. Tube 3414v operates; in. conjunction with a phase-inverter triode tube 348,. to which it is coupled through` a cathode. resistor, 348'; Grid 342 of tube 344 and grid 35i);k of tube 3156VAL are biased positively with respect to .ground by. a voltage divider comprising resistances 354 andconnected between a high voltage supply lead 358 and. ground. The anode circuits of tubes 344 and 3.46; are resistance-capacitance coupled to push-pull output tubes 362.' and' 364i` in conventional manner; this output stage being coupled through transformer 366" to the transmitter L, where the signals. are used'- to. amplitude modulate the. transmitterL carrier in the usual manner.

RECEIVING EQUIPMENT Filters O, P and Q The transmitted. signal is, received by receiver which. mayv be4 a` conventional: communication heterodyne type. After detection, the modulation components, which may be further amplied, if

necessary, by a circuit arrangement similar to amplifier K in the transmitting equipment, are Yseparated by three filters O, P and Q. The highpass filter O, in this particular application, has a cut-off frequency of 1450 cycles and the-bandlpass filter P has a pass-band of 585 to 1225 cycles per second. These two filters provide the information necessary to reconstruct the Selsyn signals, and thus, provide the angular information. The third filter Q is a low-pass f1lter with a cutoff frequency of 410 cycles per second and provides a signal the frequency of which is a function of the magnitude of the original signal S-2. Each of these filters has an attenuation of less than four decibels within its pass-band and an attenuation of at least 30 decibels in each of the other bands. In this particular case the filters may be designed conveniently with input `and output impedances of 600 ohms.

Discimz'nators R, T, and U The circuit diagram of discriminator T, which is connected to the output of band-pass filter R is shown in Figure '1; discriminators R and U are substantially the same and are shown only in block form.

The audio-frequency signal from band-pass lter P, which lies in the range between 600 and 1200 cycles, is utilized to control the repetition frequency of a square wave generator. These square Waves subsequently are differentiated and the positive peaks utilized to trigger a counter circuit which develops a direct voltage proportional to the repetition rate of the differentiated pulses.

The audio signals from filter P are applied through a resistor 368 to grid 312 of a triode tube 314 which, in combination with a similar tube 316, forms a multi-vibrator circuit. The tubes in this multi-vibrator circuit are cathode coupled through a resistance 318; anode 382 of tube 314 is coupled through a condenser 384 to grid 386 of tube 318. This circuit generates a square wave the repetition frequency of which is controlled by the frequency of the input signal fromfband-pass lter P. The frequency of oscillation when no input signal is applied is a function of the time constant of condenser 384 and resistance 388 which are adjusted, in this example, so that Vthe natural frequency of oscillation is approximately 800 cycles, i. e. near the midfrequency of the range over which the multivibrator must operate. When audio signals having greater than approximately one-tenth volt R. M. S. are appliedY to the input, the multivibrator synchronizes with the input signal and oscillates at that frequency.

The output voltage from the multi-vibrator is taken from anode 392 of tube 316 and is differentiated by a condenser 394 and a resistance 396. This differentiation produces a relatively sharp positive peak when the voltage of the square wave rises rapidly in a positive direction, and a second impulse, which is negative, when the. voltage of the square wave changes suddenly in a negative-going direction. These differentiated signals are coupled thro-ugh resistance 398 to a control grid 482 of a grid-controlled gaseous discharge tube 434.' A negative bias voltage is applied to grid 482V from a power supplyV (not shown) through a resistance 483. Anode 403 of the gaseous discharge tube is cou- .pled to a high voltage supply (noty shown) vthrough a series resistance 452, and is connected also through a resistance 4l4 to a condenser 4t?, the opposite terminal of which is connected to the ground. Cathode M8 of the gaseous tube is connected to gro-und through a variable resistance 422 which is in parallel with a capacitance 424. The output signal from the gaseous discharge tube is taken from the cathode through an R. C. filter including resistance 426 and capacitOr 425.

Tube 484 operates as a saw-tooth generator in which the capacitor 424 and resistor 422 form a storage circuit across Which a voltage is developed that is proportional to the repetition rate of the saw-tooth generator, which is triggered by the positive pulses produced by differentiating the square waves from the multi-vibrator, thus producing one saw tooth for each positive pulse; the negative pulses having no effect on the circuit. Each positive pulse, applied to grid 482, causes ignition of tube 484 which immediately transfers a fixed portion of the charge which is on condenser dit to the condenser 424 in the cathode circuit. When the charge has been transferred through the tube in this manner, the voltage on anode 488 of the tube drops to a low value, because of the voltage drop across resistance 4t2, and the tube is extinguished. Condenser lit then recharges through resistance 452 and resistance 414 to a voltage equal to the supply voltage. The tube 404 is prevented from igniting at this time by the negative bias voltage which is applied to grid 482 through resistances 486, 396, and 398. The subsequent narrow positive pulse, however, ignites the tube and causes the transfer of another identical charge to condenser 424 from condenser 4&6. The voltage across the condenser 424 is, therefore, a function of the frequency of the saw-tooth generator, that is, this voltage is proportional to the charge transferred per unit time; the same amount of charge being transferred for each cycle of the incoming signal and the discharge time constant of .1e cathode circuit remaining constant. Resistor 426 and capacitor 428 form; an R. C. filter for smoothing the output voltage. In this particular example, the values of the components so chosen that a D. C. out--` put voltage of approximately 1Q volts is developed at the mid-frequency of the frequency range with resistance 422 adjusted to its maximum value.

Inverters W and X The direct voltage from the discriminator controls the operation of an inverter circuit which develops a 60 cycle alternating voltage the magnitude and phase of which are dependent upon the magnitude and polarity, respectively, of this applied direct voltage.

With relation to the Selsyn signals the midfrequency of the channel represents a reversal in phase and` it is desirable, therefore, that the D. C. output reverse polarity at this point in order that the Selsyn signals may be properly reconstructed. This is accomplished by means of a bleeder network (Fig. '1) comprising fixed resistors 432, and 434, and a variable resistance 436 connected in series between the high voltage supply lead and the ground. A tap taken between resistances432 and 435 provides a positive bias voltage on grid 438 of tube 442 of the inverter X. In this particular example, a positive voltage of approximately 10 volts is applied to grid 438. The voltage delivered from the cathode circuit of tube 484 is connected through lead 444 to grid Q46 Of tube 448. of the inverter.

Ii.' the voltage applied to. grid fid isv so adjusted that at thel mid-frequency itis equal to the voltage applied to.r grid 4de i. e. the voltage across condenser 424, then the difference in voltage be tween the two grids will be Zero at the mid-frequency and will reverse polarity at this point. A switch 452 is provided for connecting these two grids directly together for purposes of adjustment or test.

A negative supply voltage, say 15o volts, is applied to cathodes dfl and 456 of tubes 442 and 448, respectively, through a lead 457i, the secondary winding of a transformer 458 and series resistances 452 andi 154. The transformer 45B superimposes 60 cycle alternating voltages which are 180 degrees out of phase, on the twocathodes 454 and 655. This 60 cycle alternating voltage has an amplitude, in this example, of approxin mately 0.65 volt R. M. S. Anodes it? and SS of these tubes are connected together in the output circuit.

They phase and amplitude of the output current thus depend upon the relation between the transconductances of the two triode tubes Lili?? and 448. is biased so that thistube draws no current and grid 438 of tube 4342 is biased so as to permit the flow of plate current, the output current at point 412 will have a given phase. If the biases on the two grids are reversed so that tube 448 draws plate current and tube 4t2A is cut off, the phase of the voltage appearing at point il? will be reversed. Thus, the phase ofthe output current and the amplitude are controlled by the relation ship between the bias voltages applied to the two grids. If the plate currents through the two tubesv are equal, no signal voltage appears at point 412 because the two voltages, 180 degrees out of phase, cancel. Initial balance is obtained by closing switch 452 and adjustiner the value of resistance 452 until balanced conditions are obtained. This adjustment having been made, the phase of the output current will then reverse at the point Where the potential difference applied to the two grids changes polarity.

If the characteristic curves of the two tubes are. not identical, considerable second harmonic signal may be produced by the inverter near the null point, i. e. the point of phase reversal, as a result of the imperfect cancellation caused by small differences in the plate-current grid-bias characteristics of the tubes. It is desirable to suppress this second harmonic output in order to utilize the maximum.. range. of linearity between the direct input voltage and the 6.0 cycle output "I signal. This is accomplished. by using a plate load tuned to parallel resonance at 60- cycles followed by a tuned voltage divider.

The tuned parallel circuit includes an inductance 4M and a capacitance M6, the values of which are chosen so as to produce a high-Q circuit resonant at 6.0, cycles. The output from this circuit is connected through a capacitor 418 to a voltage divider circuitY which` comprises a resistance 482 and a parallel resonant circuit having a high-Q and including an` inductance lllfl and a capacitor 48.5 andI which is resonant at 60 cycles. The output voltage is tapped 01T between the resistance 4.8.2- and thisl parallel.. resonant circuit.

The resulting amplitude; characteristic of this inverter isl linear-over a; range greater than 11G() to 1 and produces 0;;5 volt, R'. M1 S. output with a three volt signalapplied to the input.

For example, ifY grid @l-io-f tube its The di'scrimnator R1 and the. inverter W which are connected to the output or the high. pass filter Q are` identical with the discriminator inverter-just described andare illustrated in block diagram form. only.

The output oli` the low-pass. i'llter Q- isapplied to4 discriminator U which may be substantially identical with discriminator T described above; however, the output of. this discriminator may be applied directly to. a suitable indicator, as for example, recording pen V.

Amplifier Y The 5.0.1 cycle signals; delivered by inverters W and X carry theangular information provided by the original signal S-I. These signals; are applied to amplier Y (Figi. 8) whichv ampl-ies these 60 cycle signals; and reconstructs the. third Selsyn voltage which wast` not transmitted;

The signal; delivered by inverter X. is applied to. grd1li92 of triode 4&4; Anode 59s of this: tube is connected. through aV plate load resistance-4.98 to high voltage. supply lead 582. Cathode 5M of this tube is coupled to ground through a cathode bias resistance 5.8.6, which isunfbypassed to'provi'de degenerative feedback.. The: anode-496 is coupled through a. coupling. condenser 5.88 to grid. 5;!2; of triode tube dk4., the cathode resista-nce lil-il of which is. also un-bypassed providing. additional negative. feedback. The. anode 5.118, of; this tube is4 coupled also to the high voltage lead. 592. through a plate load resistance 526-. andv to, the output circuit. through.l a coupling condenser 5.22:.

The signals from. inverter W are ampliiied by tubes 52d and 526 in a similar circuit arrangement.. Small' differences in gain in the inverters and these two. stage amplifiers are compensated by'adjuszting the outputof the discriminators, so that equal; input signals tothe two discrimi'nators produce equal alternating output signals from these ampliuers;

Arr output. circuit having three tubes 528, 532, and. 53A. is. provided.' to combine the signalsv delivered by tubes 5&4. and 526 and to reconstruct the third Selsyn voltage. The signal from inverter X, after amplification by tubes #$94 and 5,.l\.4`,.is. coupled by condenser 522 to a control grid 5.35501 tube 532. The alternating output of inverter Vl, after amplification by tubes 524 and tube is coupled to grid 538 of tube` 528. Control grid 5M oi the thi-rd: tube 534 is connected to ground through a resistance 544. Cathodes. 553, 5.52?, of these tubesare connected together andi to ground` through an inductance. 555.4.

A. eedrback circuit. is provided for each of these; tubesfrom the plate-to the grid, as for example in connectionwith tube 532, anode 545 is coupled. through capacitor 5.4.8. and a variable resister 5.52; to the control grid- 535.. These negative feedbackl circuits minimize changes in ampli-cation causedr by ageing or replacement of the tubes.

With this arrangement the three Selsyn voltages originallyv present in signal. S-I are applied to the output terminals. 5.55,. 5685, andi 5152. One of thesey voltages. is. proportional; to the: input. signal voltage delivered by tube 5M, a second; voltage is proportional to the input signal voltage delivered by tube526., and. a third (developed in the unal, amplifier stage) is. proportionalk to the voltage diierencel between these input signals. rEhis latter-signal voltageis produced most readily by adjusting the circuitssso-that equal signal gain is provided by each of the three tubes. This condition will obtain provided the tubes 528, 532, and 534 are substantially identical and the plate load impedances, 514, 516, and 518 have equal impedance values. The effective driving voltage of each of these tubes, i. e. the grid-to-cathode signal voltage, will be the difference between the voltage appearing between 'the respective grid and ground and that developed across the common cathode impedance 554. In the case of tube 534, where no signal is applied directly to the grid, this will be the voltage appearing across the common cathode inductance 554, which produces an opposite effect on the plate current of tube 534 to that produced on the plate currents of tubes 528 and 532 by the signals applied to their respective grids.

The output voltages developed across the load impedances 514, 515, and 518 are proportional to the grid-to-cathode voltages of the respective tubes and are 180 degrees out of phase with respect thereto; the voltage across load impedance 518 being out of phase with the voltages across impedances 514 and 516.

The output voltage between leads 566 and 512 is equal to the algebraic sum of the voltages across plate impedance 514 and plate impedance 518, noting that these two plate impedances are connected so that the voltages produced therein by the voltage across the cathode impedance 554 cancel. This voltage is proportional, therefore, to the magnitude of the signal applied to grid 538 of tube 528. In the same manner the volt age appearing between leads 558 and 512 is proportional to the voltage applied to grid 535 of tube 532.

The voltage between leads 556 and 568 is proportional to the algebraic sum of the voltages appearing across plate load inductances 514 and 515. sition so that the output signals produced by the voltage across cathode impedance 554 cancel and the net voltage between leads 566 and 568 is proportional to the difference in voltage between the signals applied to grids 536 and 538 by amplier tubes 514 and 52B, respectively.

These signals, which have the desired relationships, are applied to the three stator windings of a synchro unit Z, which operates the turntable; the synchro rotor being connected to the same source of 60 cycle voltage that provided the alternating component in the cathode circuits of inverters W and X. Thus, the angular position of synchro motor Z and its turntable will be adjusted automatically in accordance with the position of the armature of motor D in the transmission circuit which is, of course, a function of the Selsyn signal S-l. The radial distance between the recording pen V and the center of the turntable will be in accordance with the magnitude of signal S-2. Thus, it is seen that an Ientirely practical system is provided for the transmission of angular and magnitude data and that this system may be constructed readily with ordinary components and construction techniques, and does not require especially stabilized transmitting and receiving apparatus, but may lemploy ordinary commercial type communication equipment.

We claim:

i. A method of telemetering angular information comprising the steps of generating a first signal the frequency of which is a sine function of the instantaneous angular displacement between a variable direction and a first reference These inductances are connected in oppodirection, generating a second signal the frequency of which is of a sine function of the angle of displacement between said variable direction and a second reference direction displaced 60 degrees from said rst reference direction, sirnuitaneously transmitting said signals from a iirst to a second location, deriving from said signals a first electrical quantity which is a function of the frequency of said first signal and a second electrical quantity which is a function of the frequency of said second signal, and combining said quantities to produce a third eiectrical quantity bearing a direct relationship to the angular position of said variable direction with respect to a predetermined reference direction.

2. In a telemetering system wherein a modulated radio frequency signal is radiated at one point and received at another, the method of controlling the angular position of a remotely located motor, comprising the steps of adjusting the magnitude of a first variable inductance in accordance with the sine of the angle between the angular position to be transmitted and a first reference direction, adjusting the magnitude of a second variable inductance in accordance with the sine of the angle between said angular position and a second reference direction, pro ducing a first audio signal having a frequency which is a function of the magnitude of said first inductance, producing a second audio signal having a frequency different from said first audio signal and which is a function of the magnitude of said second inductance, amplitude modulating a radio frequency carrier with said audio signals, demodulating said carrier after radiation and reception thereof, separating the two audio components of said demodulated sig` nal, producing a first direct control voltage the magnitude of which is a function of the irc quency of the first audio component, producing a second direct control voltage the magnitude of which is a function of the frequency of the second audio component, producing rst and second alternating voltages having effective magnitudes which are functions of the magnitudes of said first and second control voltages, respectively, generating a third alternating voltage having an effective magnitude which is a function of the effective magnitudes of said first and second alternating voltages, and applying said alternating voltages to a synchro motor to position, thereby the rotor thereof in accordance with the angular position represented by the adjustment of said variable inductances.

3. In a system for telemetering polar information from a first to a second location, appa,- ratus comprising a first, second, and third voltage generator producing, respectively, first, secu ond and third voltages the magnitudes of which are a function of an angle of displacement, said angle plus degrees, and said angle plus 240 degrees, a first signal generator connected to said rst voltage generator and producing a audio signal the frequency of which is a func-- tion of the magnitude of said first voltage, a second signal generator connected to said second voltage generator and producing a second audio signal the frequency of which is a function of the magnitude of said second voltage, a radiation and receiving system transmitting said signals from said i'lrst to said second location, a first and second voltage source producing, respectively, a iirst and second output voltage, a iirst regulator connected to said receiving system and regulating the magnitude of said vfirst output voltage as a function of the frequency of said first signal, a second regulator connected to said receiving system and regulating the maginitude of said second output voltage as a function of the frequency of said second signal, said rst and second output voltages having the same relative magnitudes as the first and second volt-- ages produced by said rst and second voltage generators, and an indicator connected to said Voltage sources and operated by said first and second output voltages and producing a physical. indication of said angle of displacement.

4. Apparatus for telemetering polar information from a first to a second location comprising a rst variable frequency audio signal generator including a variable inductance and producing a first alternating signal the frequency of which is a function of the value of said inductance, a control motor adjusting said irst variable inductance to a value proportional to the sine of an angle of displacement between a variable direction (which is to be telemetered) and a first reference direction, a second Variable frequency audio signal generator including a variable inductance and producing a second alternating signal which is a function of the Value of said inductance, a controlmotor adjusting said second variable inductance to a value proportional to the sine of a second angle of displacement between said direction and a second reference direction, a radio frequency carrier wave generator, a modulator connected to said first and second audio signal generators and modulating said carrier Wave with said signals, a transmitting antenna radiating the y modulated carrier wave, a receiving antenna receiving said carrier wave, a receiver connected to said receiving antenna and amplifying and demodulating said carrier wave, a first and a second filter coupled to said receiver for separating said rst and second audio signals, respectively, a first direct voltage generator connected to said rst lter and producing a rst direct voltage the magnitude of which is a function of the frequency of said first audio signal, a second direct voltage generator connected to said second filter and producing a second direct voltage which is a function of the frequency of said second audio signal, a first alternating voltage generator controlled by said rst direct voltage and producing a first alternating output voltage having a magnitude which is a function of the magnitude of said rst direct voltage, a second alternating voltage generator controlled by said second direct voltage and producing a second alternating output voltage the magnitude of which is a function of the magnitude of said second direct voltage, a third alternating voltage generator controlled by said first and second alternating output voltages and producing a third alternating output voltage having a magnitude which is a function of the magnitudes of said rst and second alternating output voltages, and a synchro motor connected to said alternating output voltages the rotor of which is thereby controlled in accordance with said angle of displacement.

5. In a telemetering system for telemetering angular information from a first to a second location through an electrical channel wherein the remote indicator is a servo-type motor in which three angularly displaced electric fields control the position of a movable rotor, apparatus for remotely controlling said servo-type motor by the transmission of only two signals comprising, in combination, an angularly movable member whose position is to be telemetered, a iirst variable frequency oscillator under the control of said member and generating a first signal Whose frequency is a function of the angle of displacement of said member from a predetermined reference direction, a second variable frequency oscillator under the control of said member and generating a second signal Whose frequency is a different function of said angle of displacement of said member, means for relaying said first and second signals to said second location, means for converting said variable frequency signals, respectively, into third and fourth constant frequency alternating signals, the magnitudes of which are functions, respectively, of the frequencies of said rst and second signals, means under the control of said constant frequency signals for producing a fifth constant frequency signal, and means coupling said third, fourth, and nfth signals to said servo-type motor thereby to control the position of its rotor in accordance with the angular position of said movable member.

OTTO H. SCHMITT. WINFIELD E. FROMM. WESLEY A. FAILS. IRA L. KASINDORF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,606,792 Isles Nov. 16, 1926 1,941,615 Mirick Jan. 2, 1934 2,179,265 Luck Nov. 7, 1939 2,319,965 Wise May 25,1943 2,379,694 Edson July 3, 1945 2,404,238 Loughlin July 16, 1946 2,405,568 Ferril Aug. 13, 1946 2,407,270 Harrison Sept. 10, 1946 2,432,772 Lear Dec. 16, 1946 2,455,618 Shepard Dec. 7, 1948 2,462,117 Mikkelson Feb. 22, 1949 

